upstream-flutter-samples/web_embedding/element_embedding_demo/web/js/demo-js-interop.js

// Sets up a channel to JS-interop with Flutter
(function() {
  "use strict";
  // This function will be called from Flutter when it prepares the JS-interop.
  window._stateSet = function () {
    window._stateSet = function () {
      console.log("Call _stateSet only once!");
    };

    // The state of the flutter app, see `class _MyAppState` in lib/main.dart.
    let appState = window._appState;

    let valueField = document.querySelector("#value");
    let updateState = function () {
      valueField.value = appState.count;
    };

    // Register a callback to update the HTML field from Flutter.
    appState.addHandler(updateState);

    // Render the first value (0).
    updateState();

    let incrementButton = document.querySelector("#increment");
    incrementButton.addEventListener("click", (event) => {
      appState.increment();
    });

    let screenSelector = document.querySelector("#screen-selector");
    screenSelector.addEventListener("change", (event) => {
      appState.changeDemoScreenTo(event.target.value);
      setJsInteropControlsEnabled(event.target.value === 'counter');
    });

    // Enables/disables the Value/Increment controls.
    function setJsInteropControlsEnabled(enabled) {
      let elements = document.querySelectorAll("#increment, label[for='value']");
      elements.forEach((el) => {
        el.classList.toggle('disabled', !enabled);
      })
    }
  };
}());